1. Field of the Invention
This invention relates to a high strength austenitic stainless steel having excellent corrosion resistance and, in particular, hydrogen embrittlement resistance in various corrosive environments.
2. Description of the Prior Art
Recently, there is a growing tendency to use high temperatures and high pressures in the chemical production processes for increasing the production efficiency. In order to go with such trend of the industry, demand is growing for stainless steel having enough corrosion resistance and strength to stand use under such high-temperature and high-pressure conditions. It has been attempted to improve corrosion resistance of austenitic stainless steel by adding Cr while precipitation of carbonitrides or intermetallic compounds has been resorted to for the improvement of strength, so that such conventional austenitic stainless steels are still unsatisfactory in corrosion resistance. Although steel strength can be increased by means of work hardening, this method involves the problems of drop of strength at high temperatures and stress corrosion cracking. There is also available a steel material such as ASTM A 269 XM-19 which is added with Mo for the improvement of corrosion resistance and Mn, N, etc., for the improvement of strength, however even such high strength austenitic stainless steel with relatively good corrosion resistance fails to remedy the problem of hydrogen embrittlement in use under the high-temperature and/or high-pressure conditions.
Thus, many attempts have been made for the improvements of stainless steel, but any of the known steel products has its own merits and demerits and there is yet available no steel which meets all the requirements to a satisfactory degree. It is therefore strongly requested in the industry to develop a new steel material which can maintain high strength as well as good corrosion resistance and, in particular, hydrogen embrittlement resistance under the strict high-temperature and high-pressure use conditions.
This invention has been deviced with the object of answering to such request. The present inventors took note of the fact that deterioration of corrosion resistance of conventional austenitic stainless steel has close relation to the presence of a heterogeneous phase (such as ferrite phase) in the steel structure and investigated the relationship between strength and corrosion resistance in the state of solid solution. It was found as a result that a steel material capable of maintaining high strength under high temperatures can be obtained by increasing the effective Cr content while also incorporating Mo and N in proper quantities in the extra low carbon steel base, and that addition of a proper amount of Ni to such extra low carbon steel can produce a prominent improving effect in resistance to pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking and, in particular, hydrogen embrittlement. It was also found that steel strength can be further enhanced without affecting said corrosion resistance by adding a suitable quantity of V in addition to Mn, N and Mo.